In a system for assisting with night-time vision, an image-intensifier tube makes it possible to deliver an image of the outside world similar to that obtained under daytime conditions. Such a system may consist of night vision sights or binoculars which are placed in front of the observer's eyes and are designed so that an intensified image of the outside world is formed on the retina.
These sights or binoculars may be independent or, for example, integrated in an aircraft pilot's helmet.
A system for assisting with night-time vision may also comprise a light-intensifier tube associated with a CCD camera and a display device.
In the known way, an image-intensifier tube generally comprises a photocathode placed behind an input window of the tube, a microchannel wafer and an output screen. It operates in the following way:
the photons corresponding to the ambient lighting pass through the input window of the tube and enter the photocathode, and cause the latter to emit electrons which are accelerated between the photocathode and the input of the wafer. PA1 the electrons are further accelerated and multiplied in the channels of the wafer by a phenomenon involving multiple collisions against the walls of the channels, PA1 at the output of the wafer, the electrons are again accelerated by the highly positive potential of the output screen and strike the latter's photoemissive layer, which causes the emission of photons and the reproduction of a visible image at the output.
Ambient luminosity which is too weak to allow visibility to the naked eye can thus be sufficient to obtain a visible image at the output of the image-intensifier tube.
The accelerations and the multiplication of the electrons in the tube are due to the application of appropriate voltages to various elements of the image-intensifier tube, such as the photocathode, the microchannel wafer and the output screen. These voltages are delivered by the electrical power supply of the tube.
An advantageous power supply has to comply with a variety of constraints which are difficult to reconcile.
Firstly, when the ambient luminosity is very weak, the power supply must allow enough light intensification to present a visible image to the operator using the system for assisting with night-time vision.
Furthermore, when the ambient luminosity is too strong, the power supply must allow protection of the tube to prevent, for example, damage to the output screen or the microchannel wafer an excessive number of, or too highly energetic electrons. In this case, the power supply may fully cut off the light intensification or very greatly reduce the gain of the tube.
In the intermediate range of values of ambient luminosity, the power supply must also make it possible to control the gain of the tube in order to intensify the light received at the input of the tube while avoiding the presentation of too bright an image to the operator at the output of the tube.
An advantageous power supply will automatically ensure, on the output screen of the intensifier tube, an image with approximately constant average illumination irrespective of the value of the ambient luminosity above a minimum threshold. It should as far as possible react quickly enough to prevent dazzle in case of an abrupt increase in the outside luminosity, and to prevent "black holes" in case of an abrupt drop in luminosity. The human eye is very sensitive to excessively strong luminosity, even over a very short time (of the order of a few hundred microseconds); it furthermore reacts slowly to adapt to an abrupt decrease in luminosity.
With current devices, and under some working conditions, the reaction time of the device when confronted with abrupt variations in luminosity is too high, on the one hand in the case of increase but, on the other hand, above all in case of decrease. These various requirements impose stringent constraints on the voltage supplies of the elements of the image intensifier.
Furthermore, an additional constraint comes from the fact that a system for assisting with night-time vision may be used, for example, by an aircraft pilot or by a land vehicle driver, and also by an infantryman. Thus, for use which does not greatly encumber the user, a system for assisting with night-time vision is generally portable and must be capable of being powered by a battery; for this reason, the energy efficiency of the power supply must necessarily be high. What is more, since the space available is very limited, bulky solutions are inappropriate.
The difficulty therefore consists in producing a power supply which meets the various constraints indicated above.
The object of the invention is to solve this difficulty with a power supply having a high-voltage regulation device for a light-intensifier tube, regulating the gain of the tube by varying the voltage of the wafer using a high-voltage amplifier whose architecture makes it possible to obtain a high reaction speed with low consumption.
To this end, an image-intensifier tube is proposed which comprises a photocathode, a microchannel amplification wafer, an output screen and a supply circuit producing a first reference voltage which is applied to a first terminal of the wafer, a second variable voltage which is applied to a second terminal of the wafer, a third variable voltage which is applied to the photocathode and a fourth voltage which is applied to the output screen, the supply circuit comprising a means for measuring the current consumed by the output screen and a circuit for controlling the second variable voltage which is applied to the wafer, this control circuit producing a fixed voltage so long as the current of the screen does not exceed a threshold and, beyond this threshold, a variable voltage slaved to the changes in the current of the output screen in one sense tending to keep this current substantially constant, the supply circuit furthermore comprising means for keeping substantially constant, beyond the said threshold, the difference between the third variable voltage applied to the photocathode and the second variable voltage applied to the wafer. The tube is characterized in that the circuit for controlling the variable wafer voltage comprises a high-voltage amplifier with transistors having low quiescent consumption and high speed in controlling the variation of the second voltage as a function of the variations in the screen current, both for increasing this voltage and for decreasing it.
Preferably, the high-voltage amplifier includes two sets of transistors in series, each connected on the one hand to the wafer, and on the other hand, to a respective supply potential. It also comprises a means for controlling the conduction of at least one of the transistors of one of the sets. The transistors of the two sets are biased so as to have low conduction in the absence of variation of the voltage of the wafer, i.e. of the second variable voltage applied to the wafer; and they are also biased so that the transistors of one of the sets on the one hand deliver current to the wafer at the instigation of the control means in order to vary the wafer voltage in one sense and that on the other hand the transistors of the other set draw current from the wafer at the instigation of the control means in order to vary the wafer voltage in the other sense.
The transistors are preferably P-channel MOS transistors in the two sets.